BGP as-path Access-list

Unanswered Question
May 30th, 2008

guys if we use permit ^$ what does that mean ??? will it allow routes or not???? cheers

I have this problem too.
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Jon Marshall Fri, 05/30/2008 - 08:19

It will allow routes from within the same AS but not from any other AS so they could be received from an IGBP session but not an EBGP session.


lamav Fri, 05/30/2008 - 09:35


That regexp (regular expression) is typically used to ensure that an AS that has connections to 2 separate Autonomous Systems (ISPs, typically) does not act as a transit AS and allow routes advertised by one ISP to be advertised over to the other.

So, AS 100 has a BGP connection to AS 200 and AS 300. As 300 advertises prefixes to AS 100, but AS 100 makes sure that it does not pass those routes, and instead advertises ONLY the local prefixes, as Jon pointed out.




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